1. Field of the Invention
The invention relates generally to roller cone drill bits for drilling earth formations. More specifically, the invention relates to thermally stable diamond inserts in roller cone drill bits.
2. Background Art
Roller cone drill bits are commonly used in oil and gas drilling applications. FIG. 1 shows a conventional drilling apparatus for drilling a wellbore. The drilling system 1 includes a drill rig 2 that rotates a drill string 3 that extends downward into a wellbore 5 and is connected to a roller cone drill bit 4.
FIG. 2 shows a typical roller cone drill bit in more detail. The roller cone drill bit includes a top end 13 threaded for attachment to a drill string and a bit body 10 having legs 14 depending therefrom, to which roller cones 30 are attached. The roller cones 30 are able to rotate with respect to the bit body 10. Cutting elements 17, 18, 19 are disposed on the roller cones 30 and are typically arranged in rows 15, 16 arranged circumferentially around the roller cones 30.
The types of loads and stresses encountered by a particular row of cutting elements depends in part on its relative axial location on the roller cone. For instance, still referring to FIG. 2, inner rows of cutting elements 15 that are located more radially proximal an axis of rotation of the roller cone than outer rows 16, 20 tend to gouge and scrape an earth formation due to their relatively low rotational velocities about the roller cone and bit axes. Thus, cutting elements 17 in the inner rows 15 on the roller cone are typically either milled teeth or inserts that are made from a softer and tougher grade of tungsten carbide that is capable of withstanding the shear stresses created from the gouging and scraping cutting action. In contrast, outer rows of cutting elements, which typically include a gage row 16 and a heel row 20 disposed at a position more proximal the leg 14, to which the roller cone 30 is attached, than the inner rows 15, tend to cut a formation through a crushing and grinding action. This cutting action subjects the gage and heel rows 16, 20 to substantial compressive loads and severe abrasive and impact wear when drilling through a hard earth formation. For these reasons, the cutting elements 18, 19 in the gage and heel rows 16, 20 are typically inserts that comprise harder grades of a tungsten carbide composite material or a superhard material such as polycrystalline diamond compact. Primary functions of the gage row cutting elements 18 include cutting the bottom of the wellbore and cutting and maintaining the wellbore diameter. Often a drill bit will become under gage due to abrasive wear of the gage row cutting elements 18. Heel row cutting elements 19 serve to compensate for this loss in bit diameter and maintain the diameter of the wellbore.
Still referring to FIG. 2, the cutting elements 17, 18, 19 may be milled teeth that are formed integrally with the material from which the roller cones 30 are made or inserts that are bonded to the roller cones 30 through brazing, sintering, or other bonding technologies known in the art, or attached to the roller cones 30 by interference fit through insertion into apertures (not shown) in the roller cones 30. The inserts may be tungsten carbide inserts, diamond enhanced tungsten carbide inserts, or superhard inserts such as polycrystalline diamond compacts.
Tungsten carbide inserts typically comprise tungsten carbide that has been sintered with a metallic binder to create a tungsten carbide composite material also known as cemented tungsten carbide. The metallic binder chosen is usually cobalt because of its high affinity for tungsten carbide. Due to the presence of the metallic binder, the tungsten carbide composite has a greater capability to withstand tensile and shear stresses than does pure tungsten carbide, while retaining the hardness and compressive strength of tungsten carbide.
Referring to FIG. 3a, a polycrystalline diamond compact (PDC) insert 300 comprises a substrate 301—that is generally cylindrical in shape—to which a polycrystalline diamond table 302 is bonded at an interface 303. The interface 303 between the diamond table and the substrate may take on various geometries, such as planar or non-planar, depending on the particular drilling application. Diamond crystals are sintered with a substrate, typically a tungsten carbide composite, and a metallic binder, typically cobalt, to form a PDC insert. The metallic binder acts as a catalyst for the formation of bonds between the diamond crystals and the substrate 301. The metallic binder also promotes bonding between individual diamond crystals (known as diamond-diamond boundaries in the art) resulting in the formation of a layer of randomly oriented diamond crystals organized in a lattice structure with the metallic binder located in the interstitial spaces between the diamond crystals. This layer 302, known as a diamond table, may also be bonded to the substrate material 301 through a brazing process, or other bonding technologies known in the art, to form the PDC cutting insert 300. The diamond table 302 is the part of the insert intended to contact an earth formation and can be formed into various geometries, including dome-shaped, beveled, or flat, depending on the given drilling application. The random orientation of the diamond crystals in the diamond table 302 impedes fracture propagation and improves impact resistance.
Although PDC inserts are typically used in connection with fixed cutter bits, they have increasingly become an alternative to tungsten carbide inserts for use in roller cone drill bits due to their increased compressive strength and increased wear resistance, as well as their increased resistance to fracture propagation resulting from shear or tensile stresses during drilling.
PDC inserts are typically subject to three types of wear: abrasive and erosive wear, impact wear, and wear resulting from thermal damage. Absent any thermal effects, volumetric wear of a PDC insert from abrasion is proportional to the compressive load acting on the insert and the rotational velocity of the insert. Abrasive wear occurs when the edges of individual diamond grains are gradually removed through impact with an earth formation. Abrasive wear can also result in cleavage fracturing along the entire plane of a diamond grain. Depending on the thickness of the polycrystalline diamond table of the PDC insert, as diamond is eroded away through contact with the formation, new diamond is exposed to the formation.
PDC inserts are also subject to thermal damage due to heat produced at the contact point between the insert and the formation. The heat produced is proportional to the compressive load on the insert and its rotational velocity. PDC inserts are generally thermally stable up to a temperature of 750° Celcius (1382° Fahrenheit), although internal stress within the polycrystalline diamond table begins to develop at temperatures exceeding 350° Celcius (662° Fahrenheit). This internal stress is created by differences in the rates of thermal expansion at the interface between the diamond table and the substrate to which it is bonded. This differential in thermal expansion rates produces large compressive and tensile stresses on the PDC insert and can initiate stress risers that cause delamination of the diamond table from the substrate. At temperatures of 750° Celcius (1382° Fahrenheit) and above, stresses on the PDC insert increase significantly due to differences in the coefficients of thermal expansion of the diamond table and the cobalt binder. The cobalt thermally expands significantly faster than the diamond causing cracks to form and propagate in the lattice structure of the diamond table, eventually leading to deterioration of the diamond table and ineffectiveness of the PDC insert.
For the reasons stated above, weight on bit (WOB) and rotary speed are carefully controlled for drill bits employing PDC cutting inserts, so as to maintain the insert contact point temperature below the threshold temperature of 350° Celcius (662° Fahrenheit). For this purpose, a critical penetrating force (vertical force component of WOB) above which the threshold temperature will be exceeded is determined, and the WOB and rotary speed are adjusted so as to not exceed the critical penetrating force. Maintaining the WOB and rotary speed of a drill bit such that the critical penetrating force is not exceeded prolongs the life of the PDC insert, but at the same time reduces the rate of penetration (ROP) of the drill bit. The heat generated from the PDC insert's contact with an earth formation can differ depending on the type of formation being drilled, and if a particular formation tends to generate very high temperatures, the viable ROP of bits with PDC inserts may be below the desired ROP and the drill bit's effectiveness severely limited.
In order to reduce the problems associated with differential rates of thermal expansion in PDC inserts, thermally stable polycrystalline diamond (TSD) inserts may be used for drill bits that experience high temperatures in the wellbore. A cross-sectional view of a typical TSD cutting insert is shown in FIG. 3b. The TSD includes a thermally stable polycrystalline diamond table 308 bonded to a substrate 306 at an interface 307. The substrate 306 may comprise a tungsten carbide composite, a diamond impregnated composite, or cubic boron nitride.
TSD may be created by “leaching” residual cobalt or other metallic catalyst from a polycrystalline diamond table. Examples of “leaching” processes may be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. In a typical “leaching” process a heated strong acid (e.g. nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid) or combinations of various heated strong acids are applied to a polycrystalline diamond table to remove at least a portion of the cobalt or other metallic catalyst from the diamond table. All of the cobalt may be removed through leaching, or only a portion may be removed. TSD formed through the removal of all or most of the cobalt catalyst is thermally stable up to a temperature of 1200° Celcius (2192° Fahrenheit), but is more brittle and vulnerable to shear and tensile stresses than PDC. Thus, it may be desirable to “leach” only a portion of the cobalt from the polycrystalline diamond table to provide thermal stability at higher temperatures than PDC while still maintaining adequate toughness and resistance to shear and tensile stresses.
TSD inserts may be used on the inner rows of a roller cone. The use of TSD inserts in the gage and heel rows of a roller cone, however, is not known in the art. Also, TSD inserts having a contoured cutting surface are not known in the art.